It is known to use an electrolyte membrane resistance calculated from an HFR (High Frequency Resistance) for the detection of a wet/dry state of a fuel cell as an example of a method for estimating a state of a fuel cell. However, since a value calculated from the HFR includes not only a pure electrolyte membrane resistance component, but also electron transport resistance components such as a bulk resistance and a contact resistance, an error has been large for the detection of the wet/dry state of the fuel cell.
Accordingly, it is proposed in Japanese Patent No. 4640661 (patent literature 1) to calculate each of a first impedance in a first frequency region corresponding to an electrolyte membrane resistance and a second impedance in a second frequency region, which is a region corresponding to the sum of the electrolyte membrane resistance and a catalyst layer resistance and lower than the first frequency region, and calculate a catalyst layer water content serving as an index of a degree of wetness of a fuel cell on the basis of a differential impedance between the second and first impedances.
Further, it is proposed in JP2013-258042A (patent literature 2) to calculate an ionomer resistance on the basis of an equation supposed from an imaginary part of a measured impedance and use this ionomer resistance as a dry/wet index of a fuel cell.